This invention relates to a semiconductor memory comprising a semiconductor substrate.
As well known in the art, a conventional random access memory which comprises a memory array on a semiconductor substrate is for use in combination with a refresh circuit for refreshing information stored in the memory array because refreshing operation is indispensable for the dynamic random access memory so as to avoid dissipation of the information in the dynamic random access memory. Such refresh operation is carried out under control of a central processing unit (CPU).
As will later be described with reference to a few figures of the accompanying drawing, another conventional dynamic random access memory has been proposed wherein a refresh circuit is arranged on the substrate together with the memory array. With this sturucture, refreshing operation is made to automatically and successively progress without being supplied with any address from an external address counter.
Such automatic refresh operation is hidden from the central processing unit. In other words, another conventional dynamic random access memory apparently looks like a static memory from the central processing unit and is therefore often called a pseudo static random access memory.
At any rate, each of the above-mentioned conventional memories is selectively operable in an active mode and a standby mode during which the memory array is enabled and disabled, respectively. Refreshing operation should be carried out even during the standby mode with electric power consumed.
An electric current is caused to flow through the substrate in each of the conventional memories and results in a variation of a potential on the substrate. As a result, the information stored in the memory array is vulnerable to destruction due to the variation of the substrate potential. In order to protect the information against destruction, each of the conventional memories comprises a substrate voltage circuit for supplying a substrate voltage to the substrate to forcibly drive the potential on the substrate to a predetermined potential. This gives rise to an increase of electric power consumption and makes it difficult to drive the conventional memory by a battery.